Named Super Sweetini, the variety has been developed by grower Bernard Sparkes of Melrow Salads.
Using 30 years of experience in growing tomatoes, Sparkes developed the variety so that it would have one of the highest levels of glutamic acid - also known as umami - which gives the variety its flavour.
Hundreds of varieties of tomato were screened for flavour over the years and the best 12 were selected to go into trials last year for the exclusively-grown product.
The tomato with the best flavour was chosen to become the Super Sweetini.
The Super Sweetini also has higher sugar levels than a standard tomato of that size and its levels of Lycopene, an antioxidant naturally present in tomatoes, have been recorded to be higher in Super Sweetini than any other tomato M&S sells.
M&S tomato expert Jo Oliver said: "We have worked tirelessly with Sparkes to develop what we think is the best-tasting tomato ever sold at M&S, and potentially on the high street."
She added: "With levels of glutamic acid higher in the Super Sweetini than any other tomato, we are sure our customers will be impressed with the amazing flavour we have created.
Oliver claimed: "The Super Sweetini will revolutionise customers' tomato-eating experience. A tomato that is so balanced in sweetness and acidity has been unheard of until now."